Stainless steel has been the predominant alloy used in orthodontic appliances. However, the nickel component of stainless steel has a tendency to leach into saliva inside the oral cavity. This may cause harmful reactions, such as allergies and the like. Approximately one in every 200 to 300 patients are known to be allergic to nickel. Thus, there has been an effort to develop orthodontic appliances that do not contain nickel.
Recently, titanium has been considered as a substitute for stainless steel. However, the use of titanium in orthodontic appliances is not new. Titanium has been used not only in dental materials, but also in medical materials since the 1950s. Because of its affinity in vivo, titanium has been used in implants and artificial joints for years. Initially, pure titanium was used, but this was not always satisfactory because even the strongest grade had a yield strength of only 500 to 650 MPa (elongation 15%).
In applications involving stress and in parts where resistance is important, Ti-6A1-4V alloys have been used, these being .alpha.+.beta. type titanium alloys. Among these, Ti-6A1-4V-ELI has often been used as a titanium alloy in vivo. Ti-6A1-4V-ELI alloy was originally developed as a refractory material by the aerospace engineering industry before being used in vivo. This alloy decreases elution of vanadium, aluminum and titanium molecules (corrosion in industrial applications) by suppressing impurities to an extremely low level. The 0.2% yield strength of Ti-6A1-4V-ELI is 800 MPa (elongation 6%) according to the ASTM Surgical Implant Standard. Such a yield strength is generally insufficient for use in current orthodontic appliances where the trend is toward miniaturization and higher yield strengths.
Currently, Ti-6A1-4V-ELI is considered the most proximate in vivo metal material. However, with this alloy there is still elution of metal ions caused by reactions with biologic saline water and organic acids in the oral cavity. The vanadium (V) component of the alloy even in minute amounts is an element that is non-degradable in the human body. Under increased local concentrations its valency increases together with oxidation which results in toxicity to the central nervous system as well as cell toxicity. Also, when the aluminum component increases in concentration because of bonding with inorganic phosphorus there is a deficiency of phosphorus in the blood and in bones.
U.S. Pat. No. 4,197,643 discloses orthodontic wires, for example, ligature wires and retaining wires, containing a .beta.-titanium alloy having the components Ti-11.5Mo-6Zr-4Sn. The alloy had a rolled (mill processed) yield strength of 17.0.times.10.sup.4 psi (1170 MPa), and after aging treatment (heat aged) for four hours at 900.degree. F. (482.degree. C.) had a yield strength of 20.1.times.10.sup.4 psi (1385 MPa). 18-8 stainless steel data shown for comparison had 27.0.times.10.sup.4 psi (1860 MPa). These properties are highly desirable in orthodontic wires such as ligature wires and retaining wires.
According to MIL standards, the 0.2% yield strength of Ti-11.5Mo-6Zr-4.5Sn in the annealed state is 620 MPa (elongation 10%). Although the standard does not clearly and specifically indicate an alloy of this composition given aging treatment after solid solution treatment.sup.1, it has on the order of 1200 MPa at 0.2% resistance and the tensile strength is on the order of 1300 MPa. Not only does an alloy of this composition not contain elements believed harmful to the human body, but it also falls generally in the range of strengths required for orthodontic appliances. However, a drawback of the Ti-11.5Mo-6Zr-4.5Sn .beta.-titanium alloy composition is that it does not have a .beta.-monophase that is stable at body temperature. FNT Solid solution treatment : STA=solution treated and aged. In this case "aged" is the same as precipitation hardening. Conditions indicated by the AMS standard are 690 to 730.degree. C. and 690 to 745.degree. C.